The present invention relates to an apparatus and method for determining and displaying functional residual capacity data and other pulmonary parameters, such as positive end expiratory pressure (PEEP) data, for patients breathing with the aid of a mechanical ventilator, such as a critical care ventilator. The invention also determines and displays relationships between these and other parameters.
Functional residual capacity (FRC) is the gas volume remaining in the lungs after unforced expiration or exhalation. Several methods are currently used to measure functional residual capacity. In the body plethysmography technique, the patient is placed in a gas tight body box. The patient's airway is sealingly connected to a breathing gas conduit connected to the exterior of the body box. By measuring lung pressures and pressures in the box, at various respiratory states and breathing gas valve flow control conditions, the functional residual capacity of the patient can be determined.
Another technique for measuring functional residual capacity is the helium dilution technique. This is a closed circuit method in which the patient inhales from a source of helium of known concentration and volume. When the concentration of helium in the source and in the lungs has reached equilibrium, the resulting helium concentration can be used to determine the functional residual capacity of the patient's lungs.
A further technique for determining functional residual capacity is the inert gas wash-out technique. This technique is based on a determination of the amount of gas exhaled from the patient's lungs and corresponding changes in gas concentrations in the exhaled gas. The gas used for the measurement is inert in the sense that it is not consumed by metabolic activity during respiration. While a number of gases may be used for such a measurement of functional residual capacity, it is convenient to use nitrogen for this purpose.
In a straightforward example in which the patient is initially breathing air, the lung volume forming the functional residual capacity of the lung will contain nitrogen in the same percentage as air, i.e. approximately 80%, the remaining 20% of air being oxygen. In a wash-out measurement, the subject commences breathing gases in which oxygen is at a different concentration than 20%. For example, the patient commences breathing pure oxygen. With each breath, nitrogen in the lungs is replaced by oxygen, or, stated conversely, the nitrogen is “washed out” of the lungs by the oxygen. While the breathing of pure oxygen could continue until all nitrogen is washed out of the lungs, in most cases, the breathing of oxygen continues until the nitrogen concentration in the exhaled breathing gases falls below a given concentration. By determining the volume of inert gas washed out of the lungs, and knowing the initial concentration of the inert gas in the lungs, the functional residual capacity of the lungs may be determined from these quantities.
Methods for determining functional residual capacity in this manner are well known and are described in such literature as The Biomedical Engineering Handbook, CRC Press, 1995, ISBN 0-8493-8346-3, pp. 1237-1238, Critical Care Medicine, Vol. 18, No. 1, 1990, pp. 8491, and the Yearbook of Intensive Care and Emergency Medicine, Springler, 1998, ISBN 3-540-63798-2, pp. 353-360. By analogy to the above described wash out measurement technique, it is also possible to use a wash in of inert gas for measurement of functional residual capacity. Such a method and apparatus is described in European Patent Publication EP 791,327.
The foregoing methods are used with spontaneously breathing patients and are typically carried out in a respiratory mechanics laboratory. But in many cases, patients that could benefit from a determination of functional residual capacity are so seriously ill as to not be breathing spontaneously but by means of a mechanical ventilator, such as a critical care ventilator. This circumstance has heretofore proven to be a significant impediment in obtaining functional residual capacity information from such patients. Additionally, the patient's illness may also make it impossible or inadvisable to move the patient to a laboratory or into and out of a body box for the determination of functional residual capacity.
It would therefore be highly advantageous to have an apparatus and method by which the functional residual capacity of mechanically ventilated patients could be determined. It would be further advantageous to associate the apparatus for carrying out the determination of functional residual capacity with the ventilator to reduce the amount of equipment surrounding the patient and to facilitate set up and operation of the equipment by an attending clinician. Such apparatus would also enable the determination of functional residual capacity to be carried out at the bedside of the patient, thus avoiding the need to move the patient.
A single determination of functional residual capacity provides important information regarding the pulmonary state of the patient. However, it is often highly desirable from a diagnostic or therapeutic standpoint to have available trends or changes in the functional residual capacity of a patient over time.
It would also be helpful to be able to relate functional residual capacity to other pulmonary conditions existing in the lungs or established by the ventilator and to changes in these conditions. For example, it is known that the pressure established by the ventilator in the lungs at the end of expiration, the positive end expiratory pressure or PEEP, affects the functional residual capacity of the lungs.
Typically, an increase in PEEP increases functional residual capacity. There are two components to the increased functional residual capacity as PEEP is increased. One component is due to stretching of the lung by the increased pressure. A second component, particularly in diseased lungs, occurs from the effect of PEEP during breathing by the patient. As a patient expires, the pressure in the lungs drops until it approaches airway pressure. As the pressure within the lungs drops, the alveoli or air sacs in the lungs deflate. If alveolar sacs collapse completely, more pressure is required upon inspiration to overcome the alveolar resistance and re-inflate the alveolar sacs. If this resistance cannot be overcome, the volume of such sacs are not included in the functional residual capacity of the patient's lungs.
By applying PEEP in the patient's airway, the additional pressure in the patient's lungs keeps more of these alveolar sacs from completely collapsing upon expiration and, as such, allows them to participate in ventilation. This increases the functional residual capacity of the patient's lungs and the increase is often described as “recruited volume.” Volume reductions are termed “de-recruitments.”
However, setting the PEEP too high can cause excessive lung distension. There may also be compression of the pulmonary bed of the lung, loading the right side of the heart and reducing the blood volume available for gas exchange. Either of these circumstances present the possibility of adverse consequences to the patient.
Still further, action such as performing a suction routine, administering a nebulized medication, or changing the ventilation parameters of the ventilator can also influence functional residual capacity and it would be helpful to be able to easily determine the effect of such actions on functional residual capacity.
An apparatus and method that would possess the foregoing characteristics and that would easily and cogently make such information available would be highly beneficial in conveniently obtaining a full understanding of the pulmonary condition of the patient and how the patient is reacting to the mechanical ventilation and to any associated therapeutic measures. The clinician could then carry out appropriate action beneficial to the patient in a timely and informed manner.